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					  Extended Validation Models
  in PKI

  Alternatives and Implications


    Marc Branchaud              John Linn
marcnarc@rsasecurity.com   jlinn@rsasecurity.com
Overview
   Existing PKI practices
   Delegated path processing
   Cross-domain delegated validation
   Implications and future directions
   Conclusions



                                         2
Existing PKI Practice: CRLs
   Original assumptions
       Online, untrusted Directory as repository
       Intermittent inter-site connectivity
       Trusted authorities (CAs) kept off-line
   Path discovery & validation is client-
    based, using data from repository and
    messages
   Limitations include timeliness, large
    volumes of data to manage and
    transport                                       3
              Traditional PKI
                 Clients do all the work

                            Client
                                      Find
                                      Path        Path        Certs
              Cert      Trusted
                                      Path     Discovery
Application




              OK?         CAs
PKI Client




                                      Path
                      Certificate     OK?         Path
                      Processing               Verification           Repository
                                      Y/N
              Yes /                    Cert
                        Policies                 Status       CRLs
               No                    status?
                                               Resolution
                                      Y/N
                                                                               4
Existing PKI Practice: OCSP
   OCSP is seeing widespread adoption
   CAs delegate to OCSP responders that
    provide signed revocation information
       Designed to enable migration from CRLs
       Preserves client-based processing model,
        many semantics
       Allows improved timeliness
       Scope constrained to revocation status, not
        full validation of certificates or paths
                                                      5
              Online Certificate Status
                 Clients no longer have to manage status

                             Client
                                      Find
                                      Path       Path        Certs
              Cert      Trusted
                          CAs         Path    Discovery
Application




              OK?
PKI Client




                                      Path
                      Certificate     OK?        Path
                      Processing              Verification           Repository
                                      Y/N
              Yes /                 Status?
               No       Policies                Status       CRLs
                                    Good /    Resolution
                                     not
                                              OCSP Server
                                                                              6
              Delegated Path Discovery
                 Clients no longer have to discover paths

              Client                                DPD Server
                                      Cert +
                     Trusted        CAs, policies      Path             Certs
              Cert     CAs                          Discovery
Application




              OK?
PKI Client




                                                                                  Repository

                                                     Status?


                                                               Good /
                                      Path




                                                                not
                     Certificate
                     Processing
                                   Verification

              Yes /                                                     CRLs
               No Policies         Path + Status      Status
                                     evidence       Resolution          OCSP        OCSP
                                                                        replies     Server
                                                                                             7
              Delegated Path Validation
                  Current DPV proposals are to offload
                   verification too
      Client                           DPV Server
                                           Path       Certs
                DPV
               Server
                                           Real
                                        Discovery
Application




                            Cert OK?
PKI Client




                Key                      Trusted                Repository
              Certificate
                                           CAs
                                           Path
              Processing
                                       Verification
                                           Real
              Trusted       Yes / No
                CAs                      Policies     CRLs
                                          Status
              Policies
                                       Resolution     OCSP        OCSP
                                                      replies     Server
                                                                           8
Delegated Path Validation
   Advantages of DPV model:
       Vastly simpler client applications
       Centralized domain administration


   Disadvantages of DPV model:
       Online  availability & security issues
       Convenient monitoring point (privacy)

                                                  9
Trust and DPV
   The DPV server is the trust anchor
       Easier to manage authority compromise
   The DPV server is the trust dictator
       Clients do not validate the server’s
        “correctness”
       Client inputs are merely hints
            Still useful for client to identify context



                                                           10
Delegating Trust Across
Domain Boundaries
   DPV servers consult other domains’
    services to build responses to queries
       Clients rely on their DPV server to select
        the right sources to validate arbitrary
        certificates
       Different DPV servers’ views may differ
   Validation combines issuer domain
    information (certificates and status)
    with RP domain policies
                                                     11
Delegated Validation Across
“Trust Fronts”
                  Issuer B control
                 B’s          Issuer
                 DPV           B CA
Relying
 Party    RP’s   A’s
          DPV    DPV
                             Issuer
Relying
                              A CA
 Party
control           A’s
                 OCSP
                   Issuer A control    12
    Forms of Delegated Validation
   Chained:
       Client gets authoritative reply via intermediary
       Intermediaries on path may be included
   Referred:
       Clients redirected to authoritative server
       Responses may be traceable to it
   Recursive:
       Each server aggregates data and generates its
        own responses
       Limited traceability                        13
DPV Implications for
Cross-Certificates
   Domains can consider inter-domain
    trust relationships in formulating their
    DPV responses
   Fine-grain activation of trust
    relationships
       Available only for some clients
       Available only in some circumstances
       Like having multiple cross-certificates
        between domains
                                                  14
DPV Implications for
Revocation
   Path construction actively involves
    intermediate domains
   Domains can consider status in
    formulating their responses
   No need to explicitly query for status
       Status is simply another factor in the
        availability of certain paths
       There is no path to a revoked certificate
                                                    15
DPV Implications for
Certificates
   Queries eventually reach the issuer
       Necessary to obtain certificate status
   Issuer can assert more than just status
       Could respond with individual certificate
        elements, e.g.:
            Subject’s DN changes after cert is issued
            Can return new DN in DPV response
   Could even return subject’s public key
       No revocation publishing at all
                                                         16
DPV Implications for
Certificates
   In the limit, certificates become obsolete
   Certificate-free PKI:
       Authorities assign identifiers to entities’
        public keys
       Entities present identifiers instead of certs
       RPs resolve identifiers to public keys via
        fully-delegated DPV
            XKMS already supports URLs for keys
   Active assertions are a new paradigm for
    PKI – X.509 didn’t consider them       17
Conclusions
   Current trend towards simplifying PKI
    clients challenges basic assumptions
   Delegating trust & distributing validation
    creates active authorities and
    intermediaries
       Introduces new issues: availability, latencies
       Facilities to constrain trust gain prominence
   Implications for revocation, certification
   Caveat adopter!                           18

				
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posted:3/5/2010
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